The effects of optical isomers of ephedrine and methylephedrine on the guinea pig tracheal smooth muscle were studied. l-Ephedrine markedly caused a graded relaxation of the guinea pig trachea where the tone had been raised spontaneously. A rightward shift of the l-ephedrine concentration-response curve was observed for propranolol and butoxamine, and the pA2 values for propranolol and butoxamine were 8.55 and 6.38, respectively. d-Methylephedrine markedly caused a graded relaxation of the guinea pig trachea contracted with histamine. Propranolol and bupranolol did not affect the relaxant response to d-methylephedrine. d-Methylephedrine competitively antagonized the contractile responses to histamine, and the pA2 value for d-methylephedrine was 5.12. These results suggest that l-ephedrine-induced relaxation of the guinea pig trachea is mediated through beta 2-adrenoceptors, whereas d-methylephedrine relaxes the guinea pig trachea by blocking histamine receptors.